A light-emitting diode (LED) is a solid state semiconductor device. The light-emitting diode comprises a light-emitting structure which comprises a p-type semiconductor layer, an n-type semiconductor layer, and a light-emitting layer disposed between the p-type semiconductor layer and the n-type semiconductor layer. The light-emitting structure comprises compound semiconductors composed of III-V group elements, such as gallium phosphide (GaP), gallium arsenide (GaAs), and gallium nitride (GaN). The theory for the light-emitting diode to emit light is that when an external electrical field is applied, the electrons from the n-type semiconductor and the holes from the p-type semiconductor are combined near a p-n junction of the fight-emitting layer. And electrical energy is converted into light energy.
FIG. 1 is a cross-sectional view of a conventional light-emitting device 1. As shown in FIG. 1, the light-emitting device 1 comprises a transparent substrate 10, a first semiconductor layer 12 on the transparent substrate 10, a second semiconductor layer 16 on the first semiconductor layer 12, and a light-emitting layer 14 disposed between the first semiconductor layer 12 and the second semiconductor layer 16. The material of the first semiconductor layer 12 comprises III-V group semiconductor material, wherein a refractive index thereof is about 3.1˜3.5. The material of the transparent substrate 10 comprises sapphire or glass, wherein a refractive index thereof is about 1.5˜1.7.
Since the difference of the refractive indexes between the transparent substrate 10 and the first semiconductor layer 12 is large, the critical angle θc between the transparent substrate 10 and the first semiconductor layer 12 is less than 35 degrees. When light emitted by the light-emitting layer 14 travels from the first semiconductor layer 12 to the transparent substrate 10, the incident angle of light must be within 35 degrees for light to exit. Light with an incident angle more than 35 degrees is totally reflected at the interface of the transparent substrate 10 and the first semiconductor layer 12 and is therefore confined to the interior of the light-emitting device 1. The light extraction efficiency of the light-emitting device 1 is reduced.
FIG. 2 is the light intensity distribution of the conventional light-emitting device 1. Since the critical angle θc between the transparent substrate 10 and the first semiconductor layer 12 is less than 35 degrees, only light with the incident angle less than 35 degrees can be extracted from the transparent substrate 10. Accordingly, the far-field angle of light emitted by the conventional light-emitting device 1 with a light intensity of 50% is about 117 degrees.